In U.S. Pat. No. 5,003,550, Welch et al. describe a monolithic MOPA device having a steerable output beam. The device includes a single mode diode laser with distributed Bragg reflector (DBR) gratings, an optical amplifier disposed in tandem with the laser and a detuned second order grating surface output coupler disposed to receive the amplified light, all formed on a common substrate. The diode laser is tunable by means of a separate tuning current I.sub.t applied to the rear DBR grating. The amplifier is a flared waveguide coupled to receive the laser output and electrically pumped to provide optical power gain to the received light. In an alternate embodiment, the amplifier is a power splitter network of branching single mode waveguides coupled at Y-junctions. The branched waveguides are followed by an array of single mode gain waveguides. The power splitter portion is pumped to compensate for scattering and splitting losses at the Y-junctions, while the single mode gain waveguides are pumped to provide an optical power gain to the lightwaves. Tuning the laser, in conjunction with the surface output coupler grating, produces a longitudinal steering of the output beam. Lateral phase controllers having an array of separately addressed electrodes may be incorporated between the amplifier output and the surface output coupler to adjust the optical path length, in order to enable compensation for lateral phase variations in the amplifier and to provide lateral steering of the output beam.
In patent application Ser. No. 08/202,359, assigned to the assignee of the present invention, Welch et al. show a monolithic, multiple flared amplifier device excited by a DBR laser.
With reference to FIG. 1, describing the structure of the aforesaid patent application, the output of a laser oscillator 11 is coupled to a first preamplifier section 19 that branches at points 59 and 61 into multiple second preamplifier sections 63, each comprising a single mode waveguide 65, with conductive contacts over the waveguides 65 for optically pumping the preamplifier sections 63. An array of flared optical amplifiers 67 optically couple to the respective outputs of the preamplifier waveguides 65. The width of each amplifier gain region 69 in the array 67 matches at an input end the width of the preamplifier waveguides 65 and increases with the light divergence toward the output facet 27. Preferably, the gain regions 69 are sufficiently wide at the facet 27 to completely fill, or nearly completely fill, the entire width of the facet 27 with the emitted light 73, with little or no dark space between adjacent light beam array elements. The facet 27 is preferably set at a nonperpendicular angle to the direction of light propagation in the amplifier array 67. In this fashion, it is possible to inject several flared amplifiers with a single laser oscillator, resulting in a high power coherent output. In addition, the branching waveguides can sometimes be designed to have equal optical path lengths in order to produce equal phase at the output facets. In other instances, the branching waveguides cannot be designed to have equal optical path lengths.
An object of the invention was to devise a MOPA array of increased power output, minimal dark space between laser elements and strong coherency.